1. Field of the Invention
The present invention generally relates to a direct conversion receiver, and especially relates to such a direct conversion receiver that is used in a cellular telephone terminal.
2. Description of the Related Art
Direct conversion type receivers have been developed in place of prior super-heterodyne type receivers for use in cellular telephone terminals.
The direct conversion system has no substantial image disturbance problem, because it directly converts RF (high frequency signal) to a base band without using an intermediate frequency stage. Because of having no IF (intermediate frequency) filter, the direct conversion system is highly promising for miniaturizing cellular telephones and reducing costs.
However, there are some problems to be solved for realizing direct conversion receivers. One of them is a DC offset problem that occurs when a received frequency is the same as the local oscillation frequency.
Ideally, since the isolation (impedance) between an LO (local oscillation signal) port and an RF port of a mixer is complete (infinite), the LO signal does not leak to the RF port in the mixer. In actual mixers, however, since the isolation is not complete, a portion of the energy of the LO signal leaks to the RF port side, and is reflected by an amplifier upstream from the mixer, returned to the mixer again, and mixed with the local oscillation signal at the mixer.
Since the RF frequency and LO frequency are the same in the direct conversion system, this component becomes DC that is called a “DC offset”. This becomes a disturbance component against a desired base band signal and may result in receiving sensitivity degradation.
The leakage is explained by using an equation. A signal input to the mixer can be represented by the following equation. The first term represents a desired frequency (only frequency ωRF component is indicated), and the second term represents local oscillation leakage (LO leakage) from the LO port to the RF port of the mixer. The coefficient A of the second term means an LO leakage coefficient.sin ωRFt+A·sin ωLOt  (a)
This can be mixed with the local oscillation signal sin ωLOt, resulting in:
                                                                                          (                                                            sin                      ⁢                                                                                          ⁢                                              ω                        RF                                            ⁢                      t                                        +                                                                  A                        ·                        sin                                            ⁢                                                                                          ⁢                                              ω                        LO                                            ⁢                      t                                                        )                                ×                sin                ⁢                                                                  ⁢                                  ω                  LO                                ⁢                t                            =                            ⁢                                                                    -                    1                                    /                                      2                    ⁡                                          [                                                                        cos                          ⁡                                                      (                                                                                          ω                                RF                                                            +                                                              ω                                LO                                                                                      )                                                                          ⁢                        t                                            ]                                                                      +                                                                                                      ⁢                                                1                  /                                      2                    ⁡                                          [                                                                        cos                          ⁡                                                      (                                                                                          ω                                RF                                                            -                                                              ω                                LO                                                                                      )                                                                          ⁢                        t                                            ]                                                                      -                                                                                                      ⁢                                                A                  /                                      2                    ⁡                                          [                                              cos                        ⁡                                                  (                                                      2                            ⁢                                                          ω                              LO                                                        ⁢                            t                                                    )                                                                    ]                                                                      +                                                                                                      ⁢                              A                /                                  2                  ⁡                                      [                                                                  cos                        ⁡                                                  (                                                                                    ω                              RF                                                        -                                                          ω                              LO                                                                                )                                                                    ⁢                      t                                        ]                                                                                                          (        b        )            
The first and third terms in the right part of the equation (b) can be removed by a LPF downstream from the mixer. The second term in the right part of the equation (b) is a desired base band signal. The fourth term in the right part of the equation (b) appears at the output of the mixer as a DC offset due to the LO leakage.
In the prior art, in order to remove DC and its neighboring frequency components and eliminate DC offset influence, a high pass filter or a capacitor for AC coupling is connected at the output of the mixer.
FIG. 1 shows a block diagram of an example of conventional cellular telephone terminals. The cellular telephone terminal shown in FIG. 1 has a transmitter 18 and receiver 20. After an antenna 10 is provided a duplexer 12, which separates transmission waves and reception waves.
As for the transmission waves, a voice signal input from a microphone 14 is encoded in a signal processor 16, modulated in the transmitter 18, and transmitted via the duplexer 12 and the antenna 10. As for the reception waves, a signal received at the antenna 10 is supplied via the duplexer 12 to the receiver 20, demodulated there, converted to a voice signal in the signal processor 16, and output at a speaker.
FIG. 2 shows a block diagram of an example of conventional direct conversion receivers. A portion surrounded by a dotted line in FIG. 2 corresponds to the receiver 20 shown in FIG. 1. Since the present invention is not related to the transmitter and the signal processor, these are not explained herein below.
In FIG. 2, a QPSK (Quadrature Phase-Shift-Keying) modulated RF signal received at an antenna 10 passes through an RF filter 102, and is amplified in an RF amplifier 103, then separated into two branch routes. The branched RF signals are mixed with a local oscillation signal (carrier) in a mixer 106a and a mixer 106b, respectively. The local oscillation signal is supplied from a local oscillator 104 and has the same frequency as that of the received RF signal. The local oscillator 104 supplies the local oscillation signal via a 90 degree phase shifter 105 to a first mixer 106a and supplies it directly to a second mixer 106b. 
From the mixers 106a and 106b are obtained base band signals (I-signal and Q-signal), which pass through low pass filters 107a and 107b respectively, so as to perform channel selection. Base band signals (I-signal and Q-signal) output from the low pass filters 107a and 107b are fed to capacitors 110a and 110b, respectively, where components in the near neighborhood of DC are removed. Signals output from the capacitors 110a and 110b are amplified by base band amplifiers 108a and 108b, respectively, and then supplied to a controller 130. High pass filters having a very low cutoff frequency can be used in place of the capacitors 110a and 110b. 
However, these conventional offset canceling methods utilizing AC coupling by capacitors or high pass filters have a problem that in a case of received signals having a DC signal component, a part of desired signals are removed resulting in degradation of receiving characteristics.
FIG. 3 is a graph of a frequency spectrum, which shows a DC offset 25 due to LO leakage, a desired base band signal 26, and frequency characteristics 27 depending on whether AC coupling or high pass filters are used.
In a case where the DC offset varies with time, a transient response is generated corresponding to time constants of the AC coupling or high pass filters. Until the transient response fades out, desired signal waves are influenced by the offset, resulting in degradation of receiving characteristics. FIG. 4(A) shows signal waves when the DC offset varies at time=t0. FIG. 4(B) shows signal waves when the DC offset is removed by AC coupling. FIG. 4 also shows a desired wave 28, a DC component 29 necessary to the system and a DC offset component 30 disturbing the system.
In order to minimize removing the desired base band signal 26 shown in FIG. 3, it is necessary to make the capacitors 110a and 110b large. However, as the capacitors 110a and 110b become larger, the time constant becomes longer.
When it is tried to cancel the DC offset by using AC coupling or high pass filters as shown in FIG. 2, if the DC offset varies with time as shown in FIG. 4(A), then the signal becomes like the dotted line 32 due to the influence of the time constants of the AC coupling or high pass filters. Therefore, there is a problem in that during the period from t0 to t1, the DC offset remains without cancellation, resulting in degradation of receiving characteristics. As the time constant becomes larger, the period from t0 to t1 becomes longer.